1. Field of the Invention
The present invention relates to a system for transmitting parallel data, and more particularly, to a transmitter system for transmitting parallel data by compensating for a crosstalk to reduce a far-end crosstalk in a receiver.
2. Description of the Related Art
Crosstalk is a phenomenon caused by electromagnetic coupling between transmission lines adjacent in parallel, and causes a timing jitter during high-speed signal transmission.
In the case of a transmission line positioned in a uniform medium like a strip line, the amount of capacitive coupling is equal to that of inductive coupling. Therefore, a crosstalk in a receiver becomes zero.
In the case of a micro strip line positioned over a printed circuit board, however, the amount of inductive coupling is larger than that of the capacitive coupling. Therefore, when data of any one transmission line transitions in a positive direction, a negative crosstalk occurs in a receiver of another transmission line adjacent in parallel.
On the other hand, when data of any one transmission line transitions in a negative direction, a positive crosstalk occurs in a receiver of another transmission line adjacent in parallel.
At this time, the magnitude VFEXT (t) of a far-end crosstalk occurring in the receiver may be expressed as Equation 1 below.
                                                        V              FEXT                        ⁡                          (              t              )                                =                                    1              2                        ⁢                          (                                                                    C                    m                                                        C                    t                                                  -                                                      L                    m                                                        L                    s                                                              )                        ⁢            T            ⁢                                                  ⁢            D            ×                                          ⅆ                                  Va                  ⁡                                      (                                          t                      -                                              T                        ⁢                                                                                                  ⁢                        D                                                              )                                                                              ⅆ                t                                                    ,                  (                                    T              ⁢                                                          ⁢              D                        =                                                            L                  s                                ⁢                                  C                  t                                                              )                                    Eq        .                                  ⁢        1            
Here, TD represents a transmission time when a signal is transmitted through a transmission line (static mode), Va represents a signal applied from a transmitter, Ls represents self-inductance, Lm represents mutual inductance between two transmission lines, Cm represents mutual capacitance between two transmission lines, and Ct represents the sum of self-capacitance and mutual capacitance.
FIG. 1 is a timing diagram of a conventional transmitter system in an odd mode, an even mode, and a static mode.
First, when independent signals are applied to micro strip transmission lines adjacent in parallel, respectively, the odd mode, the even mode, and the static mode may be defined as follows.
The odd mode correspond to a case in which the respective data of the transmission lines adjacent in parallel transition in different directions, the even mode corresponds to a case in which the respective data of the transmission lines adjacent in parallel transition in the same direction, and the static mode corresponds to a case in which any one of the data does not transition.
Referring to FIG. 1, when it is assumed that an inductive coupling coefficient is larger than a capacitive coupling coefficient, data waveforms of the receiver in the respective data modes may be described as follows.
In the odd mode, when data of a first transmission line transitions in the positive direction, data of a second transmission line adjacent in parallel transitions in the negative direction. Therefore, since the data of the second transmission line transitions in the negative direction, a positive crosstalk occurs in a receiver of the first transmission line.
In the even mode, when the data of the first transmission line transitions in the positive direction, the data of the second transmission line adjacent in parallel transitions in the positive direction. Therefore, since the data of the second transmission line transitions in the positive direction, a negative crosstalk occurs in the receiver of the first transmission line.
In the static mode, when the data of the first transmission line transitions in the positive direction, the data of the second transmission line adjacent in parallel does not transition. Therefore, since the data of the second transmission line does not transition, a crosstalk does not occur in the receiver of the first transmission line.
In the odd mode, a final signal reaches the receiver of the first transmission line fastest, due to the positive crosstalk. In the even mode, a final signal reaches the receiver of the first transmission line latest, due to the negative crosstalk.
A crosstalk induced jitter (hereinafter, referred to as ‘CIJ’) occurring at this time may be expressed as Equation 2 below.
                              C          ⁢                                          ⁢          I          ⁢                                          ⁢          J                =                                            T              de                        -                          T              do                                =                                                                      L                  s                                ⁢                                  C                  t                                                      ⁢                          (                                                                    L                    m                                                        L                    s                                                  -                                                      C                    m                                                        C                    t                                                              )                                                          Eq        .                                  ⁢        2            
Here, Tde represents a transmission time in the even mode, and Tdo represents a transmission time in the odd mode.
On the other hand, it may be assumed that the capacitive coupling coefficient is larger than the inductive coupling coefficient. In this case, in the odd mode, the final signal reaches the receiver latest, and in the even mode, the final signal reaches the receiver fastest.
A CIJ occurring at this time may be expressed as Equation 3 below.
                              C          ⁢                                          ⁢          I          ⁢                                          ⁢          J                =                                            T              do                        -                          T              de                                =                                                                      L                  s                                ⁢                                  C                  t                                                      ⁢                          (                                                                    C                    m                                                        C                    s                                                  -                                                      L                    m                                                        L                    t                                                              )                                                          Eq        .                                  ⁢        3            
As described above, in the conventional transmitter system for transmitting parallel data, the respective data reach the receiver with a time difference set therebetween, depending on whether the data transition or not and the transition directions of the data. Therefore, a timing jitter occurs in the receiver. Furthermore, the timing jitter may limit high-speed parallel transmission of data.